The present invention relates to a valve arrangement for controlling a hydraulically operated device, comprising four main control valves mounted in a bridge connection, one end point of a first diagonal of the bridge being connected to a pressure-fluid source, the other end point of the first diagonal of the bridge being connected to a storage tank, one end point of a second diagonal of the bridge being connected to the input and the other end point of this diagonal to the output of the hydraulically operated device, and a respective common pilot control being associated with each set of two of the valves for determining the direction and magnitude of the hydraulic pressure.
Control valve arrangements with check valve mechanisms cooperating in a bridge circuitry are well known. It is also known to control such valve mechanisms with the aid of a pilot slide valve whose sliding member is electromagnetically displaceable between a rest position and a plurality of operating positions. Depending on this control motion of the sliding member, communication is selectively established between one of the connections of the operated device and the associated supply check valve as well as between the other connections thereof and the associated return check valve in accordance with the desired flow direction, and a rate of flow of the pressure fluid necessary for the performance of a working step in the respective direction is adjusted in a special bypass valve mounted upstream of the bridge.
At the same time, a speed control is effected at the upstream side with the aid of the respective return check valve by bringing the latter into a metering position limiting the outflow of pressure fluid from the operated device as a function of the corresponding operational position of the sliding member also determining the adjustment of the bypass valve.
In the known arrangements, the pilot slide valve must have a certain minimum length, because of the plurality of required operational positions. The length is, in any case, sufficient to produce frictional losses or hysteresis effects which are unavoidable in view of the small manufacturing tolerances necessary for a quality control and which affect the sensibility, rapidity and accuracy of the response.
A precision machining of the great number of grooves, metering notches and slots, with which the pressure-fluid volume passing through the pilot, bypass, and return check valves must be controlled, is highly expensive. Moreover, in spite of the precision in manufacture, the thus metered flow is subjected to variations which frequently require special preventive measures in the construction, requiring additional equipment.